Pyrolytic boron nitride crucible and method

ABSTRACT

A method of forming a pyrolytic boron nitride (PBN) article and an article having layers of PBN separated by layers of PBN having a dopant of sufficient concentration to induce peeling, the steps of introducing vapors of ammonia and a gaseous boron halide in a suitable ratio into a heated furnace reactor to cause boron nitride to be deposited in layers on a substrate, with at least one gaseous dopant injected into furnace at controlled periodic interval(s) such that at least two selected layers of boron nitride are doped with said gaseous dopant(s) at a minimum average concentration of 2 atomic wt % at a depth ranging from 1000 to 2000 angstroms in each selected layer, and with the selected layers spaced apart about 0.1 micron to 100 microns apart.

This invention relates to a pyrolytic boron nitride article havinglaminated layers of anisotropic boron nitride in which a plurality ofselected layers are doped with a minimum concentration of dopantsufficient to preferentially induce peeling in one or more of theselected layers and to a method for manufacturing a pyrolytic boronnitride article consisting of laminated layers of anisotropic boronnitride in which a dopant is incorporated into a plurality of selectedlayers for preferentially inducing peeling of one or more of theselected layers relative to a given surface of the crucible.

BACKGROUND OF THE INVENTION

Pyrolytic boron nitride is formed by chemical vapor deposition using aprocess described in U.S. Pat. No. 3,182,006, the disclosure of which isherein incorporated by reference. The process involves introducingvapors of ammonia and a gaseous boron halide such as boron trichloride(BCl₃) in a suitable ratio into a heated furnace reactor causing boronnitride to be deposited on the surface of an appropriate substrate suchas graphite. The boron nitride is deposited in layers and may beseparated from the substrate to form a free standing structure ofpyrolytic boron nitride. Pyrolytic boron nitride is an anisotropicmaterial having an hexagonal crystal lattice with propertiesperpendicular to the basal plane, known as the “c-plane”, which aresignificantly different from its properties parallel to the plane, knownas the “a-plane”. Because of the high degree of anisotropy themechanical strength of the free standing pyrolytic boron nitride(hereafter “PBN”) structure is weak in the perpendicular direction whichis the direction of PBN layer growth.

Crucibles of PBN are used commercially to melt compounds at elevatedtemperatures. For example, in the production of semiconductors,crucibles of PBN are used to grow GaAs crystals. A PBN crucible is,however, subject to fracture from a build up of stress. For example whenusing the Liquid Encapsulated Czochralski method to produce singlecrystals of GaAs, fracture of the PBN crucible can occur after thecrystal is grown and a boric oxide glass B₂O₃, which is used to coverpart of the molten mass, freezes in the crucible. On freezing the boricoxide (B₂O₃) glass shrinks more than the crucible. The resultantshrinkage mismatch generates stresses within the PBN crucible that cancause it to fracture. Similarly boric oxide is often used as anencapsulant for GaAs in the vertical gradient freeze (VGF) method. Thebond between the boric oxide and the PBN surface and the shrinkagemismatch of the boric oxide and PBN on cooling causes a radial tensilestress (perpendicular to the layers) and a compressive stress (parallelto the layers) in the PBN. This causes large stresses to develop in thePBN which, in turn, can cause delamination i.e. peeling to occur orfracture. Ideally, the PBN should peel away in thin layers of controlledthickness, providing an improved and predictable service life whichwould also eliminate the risk of catastrophic failure. In the past therewas no way to predict to the number of layers that would peel off. For acrucible to have a long service life, it is necessary to control peelingand to preferably limit peeling to a single selected PBN layer ofcontrolled thickness for each crystal growth run. Moreover, the singleselected layer should peel uniformly from the body of the crucible whenthe boric oxide glass is withdrawn.

SUMMARY OF THE INVENTION

The present invention is directed to a PBN article such as a crucible oflaminated layers containing anisotropic boron nitride and to a methodfor forming a PBN crucible having laminated layers of anisotropic boronnitride in which a plurality of selected layers contain a dopant in apredetermined minimum concentration sufficient to induce peeling of oneor more selected layers in the crucible. In accordance with the presentinvention, the selected layer or layers contain dopant(s) of sufficientminimum average concentration to induce peeling and should be spaced apredetermined distance apart from one another. Preferably only one layerof PBN will peel off the crucible during each crystal growth operation.The dopant causes peeling to propagate along the layer plane such thatpeeling of the layers is uniform. The preferred spacing between theselected layers should be between about 0.1 micron and 100 microns for acrucible of given thickness to permit the number of crystal growth runsto be estimated in advance. Each of the selected layers within the PBNcrucible should be doped with an elemental dopant preferably selectedfrom the group consisting of carbon and/or oxygen, alkali metals,alkaline earth metal(s), transition metal(s), and rare earth metal(s) orselected from group 1-6 of the periodic table and/or any combinationthereof with at least one of the dopants being present at a minimumaverage concentration of 2 atomic % when measured at a depth in a rangefrom about 1000 to 2000 angstroms from the interfacial fracture surface(the surface of the peeled layer).

Broadly, the pyrolytic boron nitride article of the present inventioncomprises layers of pyrolytic boron nitride in which a plurality ofselected layers contain a dopant in a concentration sufficient to inducepeeling of one or more selected layers with the average minimumconcentration of dopant in each selected layer being above at least 2atomic % at a depth in a range from about 1000 to 2000 angstromsmeasured from the surface of the peeled layer and with each of theselected layers being spaced a predetermined distance apart of betweenabout 0.1 micron to 100 microns. Preferably only one of the selectedlayers will peel off from the crucible for each use of the crucible togrow crystals in the production of semiconductors.

A pyrolytic boron nitride crucible is formed in accordance with themethod of the present invention comprising the steps of introducingvapors of ammonia and a gaseous boron halide in a suitable ratio into aheated furnace reactor to cause boron nitride to be deposited in layerson a substrate, injecting at least one gaseous contaminant into thefurnace at controlled periodic interval(s) such that at least twoselected layers of boron nitride are doped with said gaseouscontaminant(s) at a minimum concentration level of above 2 atomic % at adepth ranging from about 1000 to 2000 angstroms in each selected layer,controlling the interval of injection to space the selected layersbetween about 1 micron and 100 microns apart and separating the cruciblefrom the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a conventional double cantilever beamtest procedure for calculating the peel strength to peel off a layer ofPBN from a sample strip of PBN material formed in accordance with thepresent invention;

FIG. 2A is a schematic drawing illustrating a “Slot Test” to identifythe planes of failure in a strip of PBN formed in accordance with thepresent invention;

FIG. 2B is a photomicrograph showing the delineation of the preferredfailure planes using the Slot Test of FIG. 2A for a strip of PBN formedin accordance with the present invention under the conditionscorresponding to Example #4 in Table 1;

FIG. 2C is a photomicrograph showing the delineation of the preferredfailure planes using the Slot Test of FIG. 2 for a strip of PBN formedin accordance with the present invention under the conditionscorresponding to Example #5 in Table 1;

FIG. 2D is a photomicrograph showing the delineation of the failureplanes for a conventional strip of PBN, not formed in accordance withthe present invention, corresponding to Example #8 in Table No. 2;

FIG. 2E is a photomicrograph of another conventional strip of PBN, notformed in accordance with the present invention, corresponding toExample #9 in Table No. 2, which did not exhibit any preferred failureplanes;

FIG. 3A is a graph showing the average dopant concentration in aselected layer of PBN corresponding to Example No. 1 in Table No's 1 and2, as a function of distance (depth) from the interfacial fracturesurface (peeled surface) of the selected layer;

FIGS. 3B and 3C are graphs showing the average concentration of amixture of carbon and oxygen in a PBN layer corresponding to Example No.5 and 4 in Table No's 1 and 2 as a function of depth measured from thesurface of the peeled layer;

FIG. 3D is a graph showing the average concentration of oxygen andcarbon in a conventional strip of PBN corresponding to Example #8 inTable No. 1 of 2, as a function of depth measured from an interfacialfracture surface;

FIG. 4 is a graph showing the correlation between the averageconcentration of dopants to the median peel strength at a given distancefrom the interfacial fracture surface; and

FIG. 5 is a graph showing the correlation of differential dopantconcentrations on peel strength at a given distance from the interfacialfracture surface.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered in accordance with the present invention that thebond strength between PBN layers in a PBN crucible can be controllablyaltered by introducing a dopant such as carbon and/or oxygen in selectedlayers to induce and control the sequence of peeling of the PBN cruciblein each of the selected layers based upon the concentration of dopantand the interval of separation. The dopants should be introduced duringthe deposition process in discrete steps or the concentration of dopantperiodically varied so as to cause an abrupt and significant change inthe concentration at selected intervals corresponding to the selectedlayers.

The PBN deposit consists of laminated layers of PBN in which selectedlayers are preferentially weakened by the presence of a dopant to inducepeeling along the plane of each selected layer in the deposit.Therefore, when a PBN crucible formed from the deposit is subjected to abuckling load (in-plane compression), radial tension or a peeling load,crack propagation will occur at the weakest point, which is along theinterface (plane of the layer). Since this interface is generated duringthe layerwise deposition process, uniform peeling can be assured.

Generally the deposition of PBN may be controlled at a constant ratewhich is modified by the inclusion of a short pulse of one or moredopant gases at selected intervals so that the selected dopant(s) willbe incorporated into the PBN at selected layers spaced a predetermineddistance apart. Alternately the deposition of PBN could be altered orinterrupted at the time dopants are introduced. The dopant(s) mayinclude any element selected from groups 1-6 of the periodic table andpreferably one or more of the following elements, C, O, Si, Al, Gaand/or alkali metal(s), alkaline earth metal(s), transition metal(s) orrare earth metals or combinations thereof. Dopants such as C and O maybe introduced by injection of such materials as CH₄, O₂, N₂O, air, CO,CO₂ or any suitable mixtures of O containing species such as water withcarbon containing species such as ethane, propane, methanol, andethanol. The choice of dopant and its concentration relative to BCl₃ isdetermined by processing conditions and application. For example, in anapplication involving Ga crystal growth, oxygen alone or in combinationwith small amounts of carbon may be injected.

Concentration of the dopant in PBN that results from the pulse isdetermined by the concentration of dopant relative to that of the boroncontaining reactive species such as BCl₃ in the gas phase. The pulseduration is determined by the desired thickness for the doped layer.Flow rate, duration and composition of the pulse have been found tocontrol mechanical properties, specifically the peel strength. The dopedinterval must be short so as to produce a controlled doped layer ofgiven thickness with the pulse duration significantly smaller than thetime intervals between the pulses so that the doped layers are distinctand spaced apart from one another by a fixed distance corresponding tothe doping interval. The process is then repeated until the totalthickness of PBN deposit is achieved. The thickness of the doped layershould preferably be in a range of 100-3000 angstroms and the thicknessor spacing between doped layers should preferably be in the range of0.0.1-100 microns, more preferably 1-75 microns and optimally 2-50microns. The doped layers are thinner by a factor of 2 to 100,000relative to the other thicker layers. The minimum average concentrationof dopant(s) in the PBN should be 2 atomic. % at a depth in the rangeabout 1000-2000 angstroms measured from the peeled surface. Morepreferably, the total minimum average concentration when more than onedopant is present such as C and O should be 3.5 atomic % at a depth ofabout 1000 to 2000 angstroms with either C or O having the desiredminimum average concentration of 2 atomic %. The thicker PBN layers mayalso contain a minority composition of components selected from one ormore of the following: Si₃ N₄, SiC, AN, TiN, C, boron carbonitride,boron oxynitride and boron oxycarbide. The thicker PBN layers may alsoshow a gradient in the composition of the minority components along thedirection of deposition.

PBN materials were formed in accordance with the present invention in acylindrical graphite furnace based CVD reactor. Reactant gases wereintroduced in a heated chamber within a water-cooled steel vacuumchamber. Graphite substrates or mandrels were placed above a nozzlethrough which reactant gases flowed into the heated chamber.Water-cooled coaxial injectors were used. Temperature was monitored byan optical pyrometer and a vacuum transducer monitored chamber pressure.Temperature and pressure were maintained at a desired level. BCl₃ andNH₃ gases were kept separate until being admitted into the hot zone. Atthe desired intervals, dopant gases (CH₄, N₂O, CO, CO ₂, etc.) weremixed with NH₃. Dopants were typically admitted for between 5 and 60seconds at intervals of 15 to 90 minutes depending upon the depositionrate. The time interval at which a pulse of dopant gas or gas mixturewas introduced was chosen so that doped layers were preferably 20 to 30microns apart. After the desired level of deposit thickness, reactivegases were turned off and furnace power was switched off. Depositionconditions are described in Table I. After cooling, PBN deposited in thetarget region was removed from the graphite substrate and analyzed forits properties. In case of Example No. 4 and 5, crucibles were producedfor their evaluation in a boric oxide melt test. Graphite mandrels wererotated around their axis to produce uniform wall thickness.

TABLE I Processing Conditions for Producing Interfacially DopedPyrolytic Boron Nitride Typical Pulse Pulse Dep. Rate Example ReferencePulse Gas Pulse Gas Interval Duration BC13 NH3 N2 Temperature Pressure(Microns/ No. Sample (slpm) (slpm) (min) (sec) (slpm) (slpm) (slpm) (C.)(mm. Hg) Hr) CO CO2 1 1N4-3-4-4 0.5 15  5 0.5 1.5 1800 0.5 100-180 21N4-3-4-6 0.5 15  5 0.5 1.5 1800 0.5 100-180 3 1N7-3-6-3 15 15 0.5 1.51800 0.5 100-180 Ch₄ N2O 4 1652B3 1.3 0.4 90 30 1.1 2 1 1730 0.4 20 UW121 5 10138t5-9 0.14 2.3 60 30 1.45 2.6 1730 0.3 30 6 N60 0 0.42 15 150.5 1.5 1800 0.5 100-180 7 N62 0.5 0.2 15 15 0.5 1.5 1800 0.5 100-180

A double cantilever beam (DCB) test as shown in FIG. 1 was used tomeasure peel strength. Beams of aluminum were used having dimensions12-13 mm wide, 50-51 mm long and about 2 mm thick. Samples were cutcarefully from a PBN deposit by using a wafering saw. A PBN plate wasused having a rectangular geometry of 12-14 mm long, 10-12 mm wide and athickness corresponding to the thickness of the deposit. In most cases,the thickness of the PBN plate was less than 3 mm. The sample was bondedbetween the aluminum beams by an epoxy resin in such a way that the edgeof the PBN closest to the applied force was normal to the beam axis soas to initiate uniform peeling from the edge. The two edges of the beamwere pulled apart at a constant speed of 2.54 mm/min in a mechanicalstrength-testing machine made by Instron. A calibrated load cellmeasured load and the value of the maximum force was obtained from theload-displacement curve. The peel strength was calculated from themaximum load, P (measured in Newton force) required to initiate crackpropagation.

The peel strength was calculated as:

S=(P/W)*(L/X)  (1)

where S is the peel strength (N/mm), i.e., it represents the effectiveforce P (N) acting at the crack front of width W (mm) to cause crackpropagation, L is the length of the cantilever beam (mm) as well as thedistance between the loading point and the opposite end of the sample,and X is the sample length (mm) along the beam. Several pieces from adeposit were tested for their strength.

The strength data was analyzed in terms of two-parameter Weibullstatistics. By plotting Ln(Ln(1/1−F)) vs. Ln S, and using linearregression value of S at a failure probability of 0.5 (or median) andWeibull modulus, m, were calculated. F is the cumulative failureprobability. These two values were used to represent strength of the PBNdeposit.

In accordance with the present invention peeling occurred along theplane of a doped layer when subjected to forces normal to the plane. Toreveal the planes of failure in a sample of PBN a “Slot Test” was usedas shown in FIG. 2, and according to the following test procedure:

1. A strip of e.g. 10-15 mm in width was cut from the deposit and thecut surfaces were smoothened by polishing with a fine grit SiC paper(typically 400 grit or finer).

2. A series of slots were cut into the strip using a wafering saw suchthat the cutting rim of the blade was made to rub against the cutsurface. Its purpose was to induce random vibrations and stresses thatwere localized to the slot being cut into the strip.

3. An optical microscope was used to examine edges of the slot that meetthe sanded surface.

4. Small hairline cracks were introduced along planes parallel to thedeposition plane as shown in FIG. 2.

5. Spacings between the cracks were compared with spacings expected fromthe deposition rate and pulse intervals. For example, if the depositionrate is 20 microns per hour and if every hour a pulse of dopant gas isintroduced for a short duration, e.g., 15 seconds, then cracks will beobserved that are spaced at 20 micron intervals.

Table I identifies seven PBN deposit examples formed in accordance withthe present invention in which peeling occurred uniformly along theinterface.

Interfacial composition was determined by x-ray photoelectronspectroscopy (XPS). Samples from peel strength tests were used todetermine surface and near-surface composition. In all cases, both ofthe new surfaces created by peeling were analyzed to determinecomposition.

Analytical services of Case Western Reserve University, Cleveland, Ohiowere used for determining dopant concentrations in the PBN depositsproduced according to this invention. For comparison, the existence ofimpurity concentrations in prior art PBN samples were determinedsimilarly. The analytical equipment used was a PHI ESCA 5600manufactured by Physical Electronics. Monochromated aluminum K-alphax-ray radiation with 800 micron aperture was used for the analysis.Incident angle for focussed x-rays was 45 degrees and take-off angle foranalyzing photoelectrons was also 45 degrees. Argon sputtering wasconducted (from the side at 45 degrees) with a sputtering rate of 42angstroms per minute. Sputtering rate was calibrated using a Ta₂O₅reference. Specimen neutralizer (electron flood gun) was set at 10% to30% of 35 volt maximum.

Standard sample preparation techniques for surface analysis experimentswere used. After analyzing the surface, depth profiles were generated bysputtering away layers of PBN and then conducting XPS analysis.Measurements were made typically at 10 to 80 angstrom intervals. Depthof a layer was determined by the sputtering rate. In all cases, carbonand/or oxygen containing species adsorbed on the surface confounded datacollected on layers up to 20-40 angstroms deep. A shift in the bindingenergy was observed when contaminated layers were removed. For example,binding energy for carbon at the surface indicated that it wascharacteristic of weakly bonded carbon. Binding energy of carbondetected on the surface after removing 20-40 angstroms showedcharacteristics similar to strongly bonded carbon such as boron carbide.Therefore data obtained at the surface (typically less than 40angstroms) was ignored.

XPS analysis gave atomic concentrations of boron, nitrogen, carbon andoxygen at a given depth. The data was used to compute averageconcentration in the layer as a function of its depth from the surface.Concentrations profiles of dopants, specifically carbon and oxygen, werecalculated to determine their distribution relative to the fracture orpeeled surface. For the purpose of comparison, average concentrations ofdopants or impurities at two different depths were determined.

If the deposit was cut carefully by a wafering saw in a direction normalto the deposition plane, then examination of edges near the cut revealeddelamination of layers. The number of layers that separated duringcutting were generally equal to the number expected from the totalnumber of pulses used for interfacial doping. In other words, byintroducing a pulse of dopants, a delamination or peeling along thatlayer was assured. FIGS. 2B-E are micrographs of some of the samplesfrom the experiments listed in Table I and of samples representing priorart 1 and 2 listed in Table II.

Table II is a compilation of the results of the surface analysis testson reference samples 1-10. Slot tests revealed that the number offracture planes contained within a PBN deposit made according to thisinvention was generally equal to the number of pulses of dopantsintroduced during the course of deposition process. Weibull statisticswas used to analyze peel strength data. In all the cases, peeling duringstrength measurement occurred uniformly along the interface.

TABLE II Results of Surface Analysis (XPS) on Peeled Surfaces, Strengthand Slot Tests Max C + O Comp. Diff. Median Peel Example Reference SideA AT Side B At Side A AT Side B At At 1000 At 1000 Strength Slot Test:No. Sample 1000 Ang. 1000 Ang. 2000 Ang. 2000 Ang. Ang. Ang. (N/mm)Layered 1 1N4-3-4-4 C:1.25 C:1.77 C:1.1 C:1.4 4 39 0.850 45; 6.31 YesO:2.29 O:2.62 O:2.24 O:2.63 2 1N4-3-4-6 C:2.3 C.1.37 C:1.77 C:1.18 4.481.140 22; 4.59 Yes O2.18 O:1.97 O:2.16 O:1.9 3 1N7-3-6-3 C:3.22 C:1.59C:2.36 C:1.43 8.98 4.650 3.,36; 17.9 Yes O:5.76 O:2.74 O:6.22 O:3.27 41652B3- C:10.14 C:8.24 C:6.14 C 5.14 10.58 1.850 3.01; 5,71 Yes UW1-21O:0.44 O.0.49 O:0.25 O:0.4 5 10138T5-9 C.1.14 C:1.21 C:0.97 C:1.05 6.132.290 2.41; 4.47 Yes O:2.7 O:4.92 O:2.68 O:5.2 6 N60 C:1.23 C:1.38 C:0.9C:1.01 6.8  2.210 3.11; 6.81 Yes O 3.36 O:5.42 O:3.37 O:6.19 7 N62C:1.09 C:1.91 C:0.85 C:1.42 8.21 4.860 2.69; 5.73 Yes O:7.12 O:1.44O:7.98 O:1.42 8 Prior Art 1 C:0.48 C:1.81 C:0.56 C:1.32 2.5  0.290 1 84;8.57 Yes, O:1.73 O:0.69 O:1.72 O:0.56 sublayers 9 Prior Art 2 C:2.49C:2.36 7.00 1.830 3.27;* No 818E O:4.51 O.2.81 10  Prior Art 2 C:1.93C:2.65 2.26 0.360 0.24;* No 818K O:5.33 O:4.25 *Strength of individualsample taken from different location in article; overall bi-modalstrength distribution A & B: A: median 0.72 N/mm, Weibull Modulus 1.93B: Median 1.75 N/mm, Weibull Modulus 2.55.

For the above example No.'s 1-7 in Table II, the number of layers offracture planes were general agreement with the number of pulsesintroduced during the deposition process. Example No. 8 (Prior Art 1) isa commercial PBN crucible available through Shin-Etsu chemicals ofJapan. The slot test revealed (See FIG. 2D) the presence of a number oflayers that re evenly spaced through the thickness. According to theproduct literature of Shin Etsu Chemicals these layers were produced asa result of varying the density of the as-deposited PBN. Example No.'s 9and 10 (labeled Prior Art 2) represent conventional PBN cruciblescommercially available from the Advanced Ceramics Corporation of Ohio.The slot test did not show any evenly spaced layers (See FIG. 2E).Similar results were obtained on a number of other commerciallyavailable PBN crucibles which were not included in Table II. As aresult, removal of PBN layers due to freezing of boric oxide during LECor in the VGF process are not predictable and may not be uniform orconsistent.

The following properties distinguish the subject invention fromconventional practice:

1. The planes along which peeling or fracture can be engineered aresubstantially parallel to one another.

2. The stress at which the layers can be separated or peeled iscontrolled by the concentration of dopants at the planes of failure.Furthermore, concentration of a particular dopant, e.g., oxygen can becontrolled by using a gas mixture having a suitable ratio of carbon tooxygen to other species, e.g., ratio of CH₄ to N₂O.

3. The number of crystal growth runs can be predicted in advance for aPBN crucible of given thickness.

The above characteristics permit a prolonged crucible life for GaAscrystal growth in a given PBN crucible during the manufacture ofsemiconductors. If the crucible is usable up to 50% of the thickness,then a two millimeter wall thickness is expected to give 50 crystalgrowth runs per crucible provided only a 20 micron thick layer isremoved during each run An article made up of layers of PBN separated bythin layers of PBN doped with elements in concentrations sufficient toinduce peeling of layers, with the average minimum concentration ofdopants in each of the selected layers being above at least 2 at. % at adepth ranging from about 1000-2000 angstroms measured from the point ofpeeling and with each of the doped layers being spaced at least onemicron apart permits for a PBN crucible of given thickness the number ofcrystal growth runs to be predicted in advance.

It should be understood that the selection of dopants and dopantconcentration may be varied during deposition of PBN to vary and controlthe strength of the layers relative to one another. This allows anarticle to resist spontaneous delamination when thickness/radius ratiois increased. It is well known that PBN, on account of its anisotropy,gives rise to significant levels of residual stresses when wallthickness to radius of curvature ratio is increased significantly. So anarticle with different radii of curvature but similar wall thicknesswould have to possess sufficiently high peel strength to resistdelamination.

It should also be understood that addition of hetero-atoms to the bulkPBN may be incorporated into this invention so as to enhance thematerial performance. For example, high concentrations of carbon alongwith oxygen can be incorporated in order to change optical properties.

It should also be understood that PBN precursor flow rates may becontinuous throughout the formation of layered structure or they may beinterrupted during the dopant introduction.

The layer peeling behavior of PBN made according to the invention wasalso tested by melting and freezing boric oxide in 6-inch diameter PBNcrucibles of the type used for growing GaAs by the vertical gradientfreeze (VGF) method.

Five boric oxide melt and freeze tests were done on each of two PBNcrucibles made according to the invention and one test using acommercially available (conventional) crucible. Some 1500 grams of boricoxide was placed in a 6-inch diameter VGF crucible and heated to 1100degrees C. for one hour and then cooled to room temperature. The boricoxide lump was then removed from the crucible and the PBN layers whichpeeled off were removed. The test was then repeated. The table belowshows the results of 5 boric oxide melt tests on each of two cruciblesmade according to the invention and on one conventionally made.

Initial Avg wt Peel thick- Crucible weight loss per ness (mm) RangeNumber Process (g) run (g) Average (mm) 138-B3 Invention 206.3 3.36 0.400.030-0.045 138-T4 Invention 190.4 2.68 0.033 0.025-0.045 1196-T2Conventional 210.0 8.08 0.095* 0.10-0.40 *based on density and averageweight loss per run

The results of peeling from the crucibles made according to theinvention are highly reproducible from run to run and the measuredaverage peel thickness is in good agreement with that calculated fromthe average weight loss per run divided by surface area and density. Incontrast, the crucible made by the conventional process peeled veryunevenly, with very thick layers in some regions and no peeling in otherareas. The average weight loss per run was also much higher in thecrucible made by the conventional process.

What we claim is:
 1. An article comprising a first plurality of layerscontaining pyrolytic boron nitride (PBN) and a second plurality oflayers containing PBN and at least one elemental dopant selected fromthe group of elements in groups 1-6 of the periodic table of elements,alkali metal(s), alkaline earth metal(s), transition metal(s) and rareearth metal(s) or combinations thereof; said second plurality of layersseparating some or all of the first plurality of layers with the dopantin the second plurality of layers being present in a sufficientconcentration to induce peeling upon a build up of internal stress alonga fracture plane in at least one of the second plurality of layers withsaid dopant being present on both sides of the fracture plane and withsaid dopant having a minimum concentration of 2 atomic weight % at adepth ranging from about 1000 to 2000 angstroms measured from thesurface of the peeled layer and with each of the layers in the secondplurality of layers being spaced apart from one another a distance of atleast one tenth of one micron.
 2. An article as defined in claim 1wherein each of the second plurality of the layers are separated adistance of about 0.1 micron to 100 microns.
 3. An article as defined inclaim 2 wherein the second plurality of layers are thinner than thefirst plurality of layers.
 4. An article as defined in claim 3 in whichthe planes along which peeling occur are substantially parallel to oneanother.
 5. An article as defined in 4 wherein the dopant is selectedfrom the group consisting of carbon and/or oxygen or mixtures thereof.6. An article as defined in claim 5 wherein the total average dopantconcentration for dopants of both C and O is above 3.5 atomic % within adistance of 1000 to 2000 angstroms from each peeled surface.
 7. Anarticle as defined in claim 6 wherein the median peel strength of eachpeeled layer lies in the range of 2-5 N/mm.
 8. An article as defined inclaim 1 wherein said first plurality of layers further includes minoritycomponents selected from the group consisting of one or more of thefollowing: Si₃N₄, SiC, AlN, TiN, C, boron carbide, boron carbonitride,boron oxynitride and boron oxycarbide.
 9. An article as defined in claim8 wherein the minority components show a gradient in composition alongthe direction of deposition such that concentration of at least oneelement will vary in excess of 0.1%.
 10. A method of forming a pyrolyticboron nitride article comprising the steps of introducing vapors ofammonia and a gaseous boron halide in a suitable ratio into a heatedfurnace reactor to cause boron nitride to be deposited in layers on asubstrate, injecting at least one gaseous dopant into the furnace atcontrolled periodic interval(s) such that at least two selected layersof boron nitride are doped with said gaseous dopant(s) to at least aminimum concentration of 2 atomic wt % at a depth ranging from 1000 to2000 angstroms in each selected layer, controlling the intervalinjection to space the selected layers from about 0.1 micron to 100microns apart and separating the article from the substrate such thatupon the application of stress in a direction substantially normal tothe plane of deposition for the boron nitride layers peeling will beinduced to occur in a doped layer along a fracture plane in which dopantis present on both sides of the fracture plane.
 11. A method accordingto claim 10, in which the flow rates of ammonia and gaseous boron halideare altered during the dopant introduction.
 12. An article composed oflayers of pyrolytic boron nitride (PBN) separated by layers of PBN dopedwith one or more elements selected from Group 1-6 of the periodic table,transition metals, rare earth metals, or combinations thereof such thatthe dopant is in a concentration in each of the doped layers above atleast 2 atomic weight % at a depth ranging from about 1000-2000angstroms measured from the surface of a peeled layer to induce peelingin each of the doped layers in a given sequence upon a build up ofinternal stress along a fracture plane in the peeled layer in whichdopant is present on both sides of the fracture plane and with each ofthe doped layers being spaced at least one tenth of one micron apart.13. An article as defined in claim 12 wherein the doped layers areseparated by about 0.1 micron to 100 microns.
 14. An article as definedin claim 13 wherein the doped layers are thinner by a factor of 2 to10,000 relative to the thickness of other PBN layer.
 15. An article asdefined in claim 14 in which the planes along which peeling occur aresubstantially parallel to one another.
 16. An article as defined in 15wherein the dopant is selected from the group consisting of carbonand/or oxygen or mixtures thereof.
 17. An article as defined in claim 16wherein the total average dopant concentration for dopants of both C andO is above 3.5 atomic % within a distance of 1000 to 2000 angstroms fromthe peeled surface.
 18. An article as defined in claim 17 wherein themedian peel strength of each selected layer is above 2 N/mm.
 19. Anarticle composed of layers of pyrolytic boron nitride (PBN) separated bylayers of PBN doped with one or more elements selected from Group 1-6 ofthe periodic table, transition metals, rare earth metals or combinationsthereof such that the dopant is in a concentration above at least 2atomic weight % at a depth ranging from about 1000-2000 angstromsmeasured from the peeled layer and with the thickness of the dopedlayers being varied relative to one another and being spaced at leastone tenth of one micron apart to cause peeling of the doped layers tooccur in a given sequence upon a build up of internal stress along afracture plane in each peeled layer in which dopant is present on bothsides of the fracture plane.
 20. An article as defined in claim 19wherein the doped layers are separated by about 0.1 micron to 100microns.
 21. An article as defined in claim 20 wherein the doped layersare thinner by a factor of 2 to 10,000 relative to that of the other PBNlayers.
 22. An article as defined in claim 21 in which the planes alongwhich peeling occur are substantially parallel to one another.
 23. Anarticle as defined in 22 wherein the dopant is selected from the groupconsisting of carbon and/or oxygen or mixtures thereof.
 24. An articleas defined in claim 23 wherein the total average dopant concentrationfor dopants of both C and O is above 3.5 atomic % within a distance ofabout 1000 to 2000 angstroms from the peeled surface.
 25. An article asdefined in claim 24 wherein the median peel strength of each selectedlayer is in the range of 2-5 N/mm.